1. Field of the Invention
The present invention relates to granular coated sodium percarbonate for detergent and its manufacturing process, and more particularly to granular coated sodium percarbonate and its manufacturing process, wherein the surface of said sodium percarbonate establishes a multiple coating layer by being internally coated with a specific composition containing alkali metal silicate while externally coated with a specific composition containing alkali metal sulfate on upper part of said internally coated layer thus providing said sodium percarbonate an excellent dissolving rate in water as well as a good storage stability, preventing it from caking during delivery or storage, and being ultimately utilized as an effective detergent bleach containing zeolite as its detergent builder.
2. Description of the Related Art
Sodium percarbonate when dissolved in water can eventually release the sodium carbonate and hydrogen peroxide, which are known to be environmentally safe and do not harm textiles when bleaching clothes and thus has been used as an oxygenated bleaching agent. The conventional powder type detergents generally contain sodium percarbonate in along with zeolite, a detergent builder, which accelerates the decomposition of sodium percarbonate. Therefore, it has been quite customary to coat the surface of sodium percarbonate with borate, silicate, sulfate or carbonate when it is used together with zeolite in order to increase stability.
With the introduction of energy conservation wave to protect the global environment, the way of wash has been also changing from hot temperature wash to either warm or cold wash and in some areas washing has been traditionally carried out at room temperature. However, if the dissolution rate of sodium percarbonate is poor the sodium percarbonate is hardly well dissolved during the wash cycle and thus either released into the sewage or remained in the clothes without exhibiting effective bleaching or sterilization. Therefore, it is shown that the dissolving speed plays a crucial role in effective utilization of sodium percarbonate.
There are roughly two different methods in manufacturing sodium percarbonate; i.e., the crystallization process and the fluidized bed spray granulation process. The crystallization process is a traditional standard wet method of that crystallizes crystal sodium percarbonate by the reaction between two solutions of sodium carbonate and hydrogen peroxide. The fluidized bed spray granulation process is a method that can increase the size of granular particles of seed by spraying said two solutions of sodium carbonate and hydrogen peroxide in fluidized bed processor by using small sodium percarbonate particles as a seed.
The crystallization process is a very complicated process that necessitates the steps of solubilization and purification of sodium carbonate in along with the granulation process to adjust the size of produced sodium percarbonate. However, the sodium percarbonate thus obtained have either non-spherical shapes or rough surface inappropriate for surface coating. In the case of the fluidized bed spray granulation process, hot blast of gas is essential to evaporate huge amount of water and the high energy consumption makes it uneconomical.
GB Pat. No. 1,538,893 disclosed a traditional coating method to increase the stability of sodium percarbonate by mixing sodium sulfate and sodium silicate. Sodium silicate is an effective coating material used in increasing the stability of sodium percarbonate, however, it has a low dissolving speed in water generally and also shows a caking during the delivery or storage. Further, when sodium silicate, as a coating material, is located at the outermost of the coating films, i.e., exposed on the surface, the caking becomes more accelerated. Sodium carbonate can also cause sodium percarbonate to be caked, and the single coating mixed with sodium carbonate, sodium sulfate and sodium silicate as disclosed in the GB Pat. No. 1,538,893 has the disadvantage of generating the caking as sodium silicate and sodium carbonate are exposed on the surface. In the case of the coated sodium percarbonate disclosed in the GB Pat. No. 1,538,893, the stability of said sodium percarbonate is worse than that of sodium perborate and the improvement in stability is also not noticeable as compared to uncoated sodium percarbonate. Therefore, the single coating method as described in the GB Pat. No. 1,538,893 is not desirable because of the ineffectiveness in increasing stability. Moreover, the current powder detergents distributed in the market contain zeolite which is known to accelerate the decomposition of sodium percarbonate, and thus the sodium percarbonate disclosed in the GB Pat. No. 1,538,893 is not recommended to use.
The drawbacks of coated sodium percarbonate are also mentioned in other prior art; i.e., U.S. Pat. No. 5,935,708 described that the coated sodium percarbonate showed a caking because the outermost coating layer of the coated sodium percarbonate consists mainly of sodium carbonate, and U.S. Pat. No. 5,902,682 pointed out that coated sodium percarbonate in general have a rapid decomposition of active oxygen and thus multi-coating is more preferred. U.S. Pat. Nos. 5,462,804 and 5,902,682 both disclosed the method to improve the stability of sodium percarbonate by coating in common with silicate, magnesium sulfate, and alkali metal salts such as carbonate, bicarbonate, and sulfate in a sequential order or simultaneously. Adding silicate or carbonate to magnesium sulfate generates insoluble magnesium silicate (MgSiO3) or magnesium carbonate (MgCO3) and once said insoluble coating films are formed on the surface of sodium percarbonate they can block the permeation of moisture into sodium percarbonate. However, said insoluble coating films are water-insoluble and thus procrastinating the rate of dissolution of sodium percarbonate. U.S. Pat. No. 5,462,804 mentioned about the effect of improved dissolution rate but the level of increased dissolution rate cannot be measured because the specific temperature to measure the rate of dissolution is not identified, and U.S. Pat. No. 5,902,682 does not describe anything about the rate of dissolution. Both of the above-mentioned U.S. Pat. used silicate and magnesium sulfate as coating substances thus resulting the low dissolution rate of sodium percarbonate. In addition, the outermost coating layer consists of either carbonate or silicate, or the mixture of carbonate, silicate, and magnesium sulfate, all of which incur the caking during the storage of sodium percarbonate. Caking can take place both in a packing container and a storage tank of sodium percarbonate., and the caking makes the free flowing and convey of sodium percarbonate quite troublesome.
U.S. Pat. No. 5,714,201 disclosed a method of restricting the modulus of SiO2/Na2O of sodium silicate used in coating or manufacturing of sodium percarbonate to 1.5-2.3 in order to improve the dissolving speed. The above-mentioned modulus of sodium silicate can much influence the dissolution rate)of sodium percarbonate, and in general the greater the modulus of SiO2/Na2O, the slower the rate of dissolution of sodium percarbonate.
Here, the rate of dissolution of sodium percarbonate is influenced by the modulus of SiO2/Na2O if the content of sodium silicate is more than 1 wt. % of sodium percarbonate, however, there is almost no influence by the modulus if the content of sodium silicate is less than 1 wt. %. Moreover, the sodium percarbonate is well dissolved when manufactured using sodium silicate having low modulus of SiO2/Na2O of sodium silicate because the hydrophilic Na2O content increases; however, the sodium percarbonate absorbs moisture from the environment when stored for long period of time and results in caking. Therefore, it is more preferred to reduce the content of sodium silicate rather than adjusting the modulus of SiO2/Na2O of sodium silicate to enhance the dissolving speed of sodium percarbonate. The U.S. Pat. No. 5,714,201 only focused on the increase of dissolving speed of sodium percarbonate according to the restricted modulus of SiO2/Na2O of sodium silicate and there was nothing described about the long-term storage stability of sodium percarbonate according to the modulus of SiO2/Na2O of sodium silicate.
U.S. Pat. No. 5,935,708 suggested a method to coat sodium percarbonate which was produced by fluidized bed spray granulation method to improve the stability of sodium percarbonate by using sodium sulfate alone. This method is advantageous in that it can provide a good dissolution rate for the sodium percarbonate because it does not use sodium silicate, however, the stability cannot be achieved by using sodium sulfate alone when the particles of sodium percarbonate are small.
In the method of coating sodium percarbonate by using sodium sulfate alone, the increase in stability in fact relies on the size of the sodium percarbonate particles rather than its manufacturing method, and the stability cannot be expected to increase by this method when the diameter of the sodium percarbonate particle is less than 0.6 mm. U.S. Pat. No. 5,935,708 does not disclose the diameter of the sodium percarbonate particles manufactured by the conventional crystallization method. In order to achieve the stability of the sodium percarbonate particles less than 0.6 mm in diameter, the coating content of sodium sulfate should be more than 10 wt. %, however, the increase coating content of sodium sulfate would also incur the loss of a great deal of active oxygen.
U.S. Pat. Nos. 5,532,518 (boric acid), 5,366,655 (Na-borate, metaborate), 5,556,834 (boric acid), 5,658,873 (tetraborate, pentaborate), 5,665,427(boric acid), 5,670,470 (boric acid) disclose coating methods by using boron compounds, however, the boron compounds are not appropriate as a coating material because they are not health-oriented nor environment-compatible.
U.S. Pat. Nos. 4,120,812 (PEG), 5,258,133 (Na-citrate) and 5,336,433 (fats, waxes) disclosed methods to coat the sodium percarbonate by using organic compounds. Further, organic compounds are more expensive than inorganic salts and the coated organic layer is oxidized by the sodium percarbonate and eventually degraded thus resulting in the decrease in the stability.
The present invention relates to a coated granular sodium percarbonate for detergent and its manufacturing process, and more particularly to a coated granular sodium percarbonate and its manufacturing process, wherein the surface of said sodium percarbonate establishes a multiple coating layer by being internally coated with a specific composition containing alkali metal silicate while externally coated with a specific composition containing alkali metal sulfate on upper part of said internally coated layer thus providing said sodium percarbonate an excellent dissolution rate in water as well as a good storage stability, preventing it from caking during delivery or storage, and being ultimately utilized as an effective detergent bleach containing zeolite as its detergent builder.
The object of the present invention is to manufacture granular coated sodium percarbonate which is stable to be used as a detergent composition, has a superior dissolution rate and can prevent caking phenomenon during delivery and storage.
The above-mentioned coated sodium percarbonate is characterized by whose surface being multi-coated with both inner layer and outer layer, wherein said inner layer comprises mixture of alkali metal silicate and at least one compound selected from sulfate, carbonate, and bicarbonate of alkali metals and said outer layer comprises mixture of alkali metal sulfate and at least one compound selected from carbonate and bicarbonate of alkali metals. In particular, the alkali metal silicate which comprises the inner layer is kept to be less than 1 wt. %, preferably 0.3-0.5 wt. %, in order to prevent the caking as well as the delay in dissolving rate. In prior art, it had been very common to use magnesium salts such as magnesium sulfate or magnesium chloride when using silicate as a coating material, however, said magnesium salts are now shown to deter the dissolving rate and thus the use of said magnesium salts are avoided in the present invention.
To manufacture uncoated sodium percarbonate, anhydrous sodium carbonate is continuously stirred in a reactor while aqueous hydrogen peroxide is being sprayed to form the sodium percarbonate particles. Here, to prevent the increase in temperature of reacting mixtures by exothermic reaction and also to prevent said reacting mixture from being got too wet, cold air is introduced into the reactor upon the initiation of the reaction to adjust the temperature (20-50xc2x0 C.) and the water content (5-20%) of the reactant.
The constant introduction of cold air into the reactor while stirring anhydrous sodium carbonate in the reactor results in the formation of sodium percarbonate particles having a relatively larger size in diameter.
Thus obtained slightly wet sodium percarbonate particles are continuously passed into the fluidized bed dryer and are produced into dry sodium percarbonate.
Here, the particles either too big or too small are sorted out by a sieve screen while the recovered one are recharged into the reactor after pulverizing rather big particles. Then, sodium percarbonate containing more than 14 wt. % of active oxygen is produced without loss of hydrogen peroxide. Then, a certain amount of stabilizers are added as in conventional methods and U.S. Pat. No. 5,851,420 describes the stabilizers and the reaction conditions in more detail in the Examples. Thus obtained uncoated sodium percarbonate is then fluidized in the fluidized bed coater by means of fluidizing hot gas and the aqueous coating solution is sprayed along with air through a nozzle to generate a coating layer on the surface of sodium percarbonate. The coating solution can be sprayed counter-currently or co-currently with respect to the introduced fluidizing hot gas, typically air or inert gas, or alternatively in both directions.
The present invention can be delineated in more detail as follows. In manufacturing coated granular sodium percarbonate, alkali metal silicate used as an inner coating material is called according to the modulus of SiO2/metal oxides (M2O) as orthosilicate, metasilicate, disilicate or water glass, and they can be all used as coating materials of sodium percarbonate. If sodium percarbonate is produced by using alkali metal silicate with low modulus of SiO2/metal oxides (M2O) the resulting sodium percarbonate would have a good dissolution rate in water, however, its stability will be decreased and caking will be increased. On the other hand, if sodium percarbonate is produced by using alkali metal silicate with high modulus of SiO2/metal oxides (M2O) the resulting sodium percarbonate would have a poor dissolution rate in water, however, its stability will be increased.
Considering the characteristics of alkali metal silicate, the present invention produced a mixed composition comprising at least one compound selected from sulfate, carbonate, bicarbonate of alkali metals along with alkali metal silicate, which is known to have a good dissolution rate and a good stability. When the content of alkali metal silicate is more than 1 wt. % of sodium percarbonate, the dissolution rate increases as the modulus of SiO2/metal oxides (M2O) decreases. However, the extent that the dissolution rate can be improved is rather restricted and the rate of dissolution generally becomes 3-5 times slower than that of uncoated sodium percarbonate. When the content of alkali metal silicate is less than 1 wt. % of sodium percarbonate, the difference in dissolution rate according to the modulus of SiO2/metal oxides (M2O) is negligible, and alkali metal silicate is preferred to have the modulus of SiO2/metal oxides (M2O) of 2.5-3.5 considering that low modulus of SiO2/metal oxides (M2O) usually leads to the decrease in the stability. The dissolution rate and the stability can be improved when alkali metal silicate is mixed with at least one compound selected from sulfate, carbonate, and bicarbonate of alkali metals. When coating small amount of materials on the surface of sodium percarbonate, spraying of highly diluted coating materials would help to coat the surface rather evenly, however, it would require longer coating time and result in much energy consumption due to higher amount of water that needs to be evaporated. But applicants were surprised to discover that the content of alkali metal silicate is less than 1 wt. % of sodium percarbonate in coating the surface of sodium percarbonate, mixing alkali metal silicate with at least one compound selected from sulfate, carbonate, and bicarbonate of alkali metals would be able to provide improved stability and would have a good dissolution rate. Therefore, the present invention by mixing alkali metal silicate with at least one compound selected from sulfate, carbonate, and bicarbonate of alkali metals, could manufacture a coated sodium percarbonate which resolved the drawback of poor dissolution rate of the conventional ones while still enabling to maintain the improved stabilizing effect due to the addition of alkali metal silicate. To achieve both the good stability and the good dissolution rate have not been appeared very feasible; for example, U.S. Pat. Nos. 5,462,804, 5,714,201, and 5,902,682 used alkali metal silicate as a coating material for the improvement of stability but the resulting products had prolonged dissolution rate, whereas U.S. Pat. Nos. 4,105,827, 5,346,680, and 5,935,708 which did not use alkali metal silicate for the improvement of dissolution rate showed decreased stability of the products.
In the present invention, the alkali metal silicate as an inner coating material is preferred to be less than 1 wt. % of sodium percarbonate, more preferably to be 0.3-0.5 wt. %. Another coating material, i.e., at least one compound selected from sulfate, carbonate, and bicarbonate of alkali metals, is preferred to have its weight content based on anhydrous form being 1-3 times of that of alkali metal silicate. The total content of inner coating materials comprised as such is preferred to be 1-2 wt. % of anhydride-based sodium percarbonate.
Considering the above-mentioned alkali metal silicate used as an inner coating material caused caking, the coating of sodium percarbonate using alkali metal silicate is preferred to use multi-coating composed of at least two layers, and it is generally preferred that the inner coating layer is composed of alkali metal silicate while the outer coating layer is composed of a coating composition that can prevent the caking resulted from the alkali metal silicate. Therefore, the present invention used the mixture of alkali metal sulfate along with at least one compound selected from bicarbonate or carbonate of alkali metals as an outer coating material. That is, by adding an outer coating composed of alkali metal sulfate and the like on upper part of the inner coating layer, the present invention resolved the caking occurring during the delivery and storage of sodium percarbonate.
Using bicarbonate or carbonate of alkali metals as an outer coating material excluding alkali metal sulfate would not help to prevent the caking but would rather speed up the caking process instead. Adding alkali metal silicate in addition to the above-mentioned outer coating material can also help to alleviate the caking process but the effect may be negligible. Therefore, using alkali metal sulfate along with at least one selected from bicarbonate or carbonate of alkali metals in an appropriate ratio as an outer coating materials in order to improve the stability of sodium percarbonate as well as to prevent said caking process. The extent that the stability is increased is shown to be more obvious when the above-mentioned materials are used together compared to when they are used independently. Of the outer coating material in this invention, the amount of bicarbonate or carbonate of alkali metals is adjusted to take 10-25 wt. % of the total anhydride-based content of outer coating material. There would occur the caking of sodium percarbonate if the amount of bicarbonate or carbonate of alkali metals is more than 25 wt. % of the total outer coating amount, whereas the preventive effect will be completely lost if it is over 50 wt. % of the total outer coating amount. If the amount of bicarbonate or carbonate of alkali metals is less than 10 wt. % of the total outer coating amount, the synergistic effect generated by combining with alkali metal sulfate would be diminished. The total amount of anhydride-based outer coating is preferred to be approximately 4-8 wt. % of sodium percarbonate. If said amount is less than the above range, the improvement in stability is ineffective and dissolution rate is also decreased, whereas the amount of active oxygen content of coated sodium percarbonate becomes lower if said amount is relatively high. In U.S. Pat. No. 4,105,827 and GB Pat. No. 1,538,893, where alkali metal sulfate and alkali metal carbonate were used together, it appears that the relatively higher percentage of alkali metal carbonate resulted in the caking.
Both the inner and outer coating solutions can contain surfactants. Surfactants can lower the surface tension of coating solution so that the coating solution sprayed from the spray nozzle can be widely spread to produce multiple thin coating layers with rigidity upon contact with the surface of fluidizing sodium percarbonate particles, which would then help to increase the stability of said sodium percarbonate. The preferred amount of surfactant is 0.1-1 wt. % of the total coating solution and if the amount of surfactant is more than 1 wt. % the dissolution rate tends to be deteriorated. Anionic and nonionic surfactants can be both used.
In this invention, the sodium percarbonate to be coated becomes moved around by means of a fluidized bed coater which introduces fluidizing gas, typically air, from the lower part. Each coating solution is prepared as an aqueous solution so that it has 10-30 wt. % of concentration and is sprayed through one or more nozzles to either co-currently or counter-currently, or alternating in both directions relative to the fluidizing gas while maintaining a constant temperature and eventually becomes adhered to the surface of sodium percarbonate. The gas temperature used to fluidize sodium percarbonate is 80-130xc2x0 C. and extreme care should be taken so that the temperature do not to go over 130xc2x0 C. because the sodium percarbonate will be decomposed beyond that temperature. It is also important to keep the temperature of sodium percarbonate particles at 30-70xc2x0 C. when spraying coating solutions by adjusting both the amount of coating solution and the air influx. Since the inner and outer coating solutions do not react with each other they can be sprayed using the same nozzle and upon completion of inner coating there needs a 2-5 min of drying which is then followed by outer coating. When all the coating procedure is completed, there will be another drying period of 4-10 min to get rid of all the remaining water content which would then result in the production of stable coated sodium percarbonate particles without loss of active oxygen during the coating process.